1. Field
The exemplary embodiments described herein relate to computed tomography (CT) systems. In particular, exemplary embodiments relate to a multilayer energy-integrating detector.
2. Description of the Related Art
The X-ray beam in most computed tomography (CT) scanners is generally polychromatic. Yet, third-generation CT scanners generate images based upon data according to the energy integration nature of the detectors. These conventional detectors are called energy-integrating detectors and acquire energy integration X-ray data. On the other hand, photon-counting detectors are configured to acquire the spectral nature of the X-ray source, rather than the energy integration nature. To obtain the spectral nature of the transmitted X-ray data, the photon-counting detectors split the X-ray beam into its component energies or spectrum bins and count the number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Since spectral CT involves the detection of transmitted X-rays at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Spectral CT is advantageous over conventional CT because spectral CT offers the additional clinical information included in the full spectrum of an X-ray beam. For example, spectral CT facilitates in discriminating tissues, differentiating between tissues containing calcium and tissues containing iodine, and enhancing the detection of smaller vessels. Among other advantages, spectral CT reduces beam-hardening artifacts, and increases accuracy in CT numbers independent of the type of scanner.
Conventional attempts include the use of integrating detectors in implementing spectral CT. One attempt includes dual sources and dual integrating detectors that are placed on the gantry at a predetermined angle with respect to each other for acquiring data as the gantry rotates around a patient. Another attempt includes the combination of a single source that performs kV-switching and a single integrating detector, which is placed on the gantry for acquiring data as the gantry rotates around a patient. Yet another attempt includes a single source and dual integrating detectors that are layered on the gantry for acquiring the data as the gantry rotates around a patient. All of these attempts at spectral CT were not successful in substantially solving issues, such as beam hardening, temporal resolution, noise, poor detector response, poor energy separation, etc., for reconstructing clinically viable images.
Spectral CT also has the following short-comings. Slow kV-switching presents a problem during patient movement and when dynamic changes are made between scans. Decomposing the data domain is also very difficult. Fast kV-switching is required for high sample rates to overcome the movement problem. However, a complex and costly system is required, which still does not provide ideal waveforms, and there is a poor noise balance with spatial registration.
Dual-layer detectors have a limited energy separation and are also more costly than a single-layer detector. In addition, they are not adaptable to patient and scan conditions. Dual-source imaging systems have a problem with data domain registration, which requires the less-useful image domain spectral decomposition method, and the system is much more costly than a single-source imaging system.
Photon-counting CT systems are not ideal for general purpose, diagnostic, or clinical CT. At the high count rate required for clinical CT, pile-up and polarization present problems. The spectroscopic accuracy of the readings is questionable, due to energy sharing and K-escape. A photon-counting system is also more costly.